Sediment oxygen demand (SOD) was measured in two shallow,
interconnected lakes in southern Oregon, Upper Klamath Lake and Agency
Lake, in spring and late summer of 1999. Upper Klamath Lake contains
populations of two endangered fishes, the shortnose sucker and the Lost
River sucker, and low dissolved oxygen concentrations in summer are
thought to be one factor affecting sucker populations.

The distribution of SOD20 values (measured sediment
oxygen demand values corrected to 20o C [degrees Celsius])
had a median value of 1.6 g/m2/day (grams per square meter
per day) in the spring and 1.7 g/m2/day in the late
summer. These values were well within the range of values in the
literature for sites with similar sediment characteristics: primarily
silty with at least a moderate amount of organic content.

Little variation in SOD was observed--the interquartile range in
values was 0.4 g/m2/day in the spring and 0.7
g/m2/day in the late summer. A significant exception was
apparent in Ball Bay, where SOD in the late summer was greater than
10.2 g/m2/day. In the absence of primary production, an SOD
of this magnitude could deplete the water column of oxygen in a few
days. This measurement provided evidence that localized areas of very
high SOD occur episodically in the bays, perhaps associated with large
algal mats being trapped by the lake circulation patterns.

A statistical test for a spring to late summer difference in the
median values of SOD confirmed that SOD in the late summer (median
value 1.7 g/m2/day) was significantly higher than in the
spring (median value 1.2 g/m2/day). The difference was
primarily due to seasonal changes in temperature, however; when SOD
values were corrected to 20o C, there was no seasonal
difference in the median values.

There was no correlation between SOD20 and the
sediment characteristics measured in this study: percent fines, organic
carbon, and residue lost on ignition.